Satellite navigation is based on a global network of satellites that transmit signals from several thousand miles in earth orbit. A number of different Global Navigation Satellite Systems (GNSS) have either been launched or are in advanced stages of development. For example, a Galileo is an example of a GNSS system which will ultimately provide a highly accurate, guaranteed global positioning service under civilian control. It is anticipated that Galileo will be inter-operable with other global satellite navigation systems such as American-based Global Position System (GPS) and the Russian-based Global Orbiting Navigation Satellite System (GLONASS).
In a GNSS such as Galileo (when available), a user is able to use his or her receiver to compute the receiver's a position using any of the satellites in any combination. It is anticipated that a system such as Galileo will be capable of delivering real-time positioning accuracy within a matter of meters.
New and proposed GNSS systems, such as Galileo, as well as a modernized GPS, will use two separate signals. One signal, referred to herein as the pilot signal, is used to provide good acquisition and tracking properties. The other signal, referred to herein as the data signal, is used to carry the satellite data message. GNSS receivers need to possess the ability to receive both signals in order to operate satisfactorily. The pilot signal is used for efficient acquisition and low signal-to-noise ratio (SNR) tracking. The data signal is used to receive the satellite data message that is required for navigation. It should be noted that, while the receiver needs the information contained in the satellite data message, it does not matter how the information is obtained. For example, an aided GNSS system may obtain the data using other forms of communication. However, if the receiver is intended for standalone use (i.e., without any aiding sources), then it does need to obtain the data message when in standalone operation. Standalone capabilities are often required features of GNSS receivers.
If the GNSS system at issue has separate pilot and data signals, then the GNSS satellites transmit the pilot and data signals on a common carrier frequency. The pilot signal and the data signal are designed so that the receiver can distinguish between the two signals. The GNSS receiver needs to be able to receive two signals from each satellite that is used for the particular transmission, although the reception does not need to be simultaneous. However, the need to receive two separate signals makes the required hardware for the receiver bigger. The hardware size is also directly tied to the product cost. To solve this problem, more efficient receiver architectures are needed which minimize the hardware complexity.
Conventional GNSS systems do not utilize separate pilot and data signals. Currently, GPS coarse/acquisition (C/A) code and precise/encrypted (P/Y) code could be considered to be somewhat similar to pilot and data signals, respectively, but such arrangements are not generally available for commercial GPS receivers. In the past, P/Y-code capable GPS receivers have implemented completely separate hardware for reception of the C/A code and the P/Y code. However, this involves essentially duplicating the hardware involved, which increases the product complexity and cost. Although some military-grade receivers can directly acquire the P/Y code without utilizing C/A code, this requires even more expensive receiver hardware.
FIG. 1 shows a typical conventional GNSS receiver correlation channel that is used to track one satellite signal (either a data signal or a pilot signal). There are several variations for achieving a smaller hardware configuration or a higher performance level. FIG. 2 shows how two such channels can be used to use both pilot and data signals from one satellite, thus duplicating the hardware size needed for each satellite. In this arrangement, a correlator channel for the pilot signal is used for measurement data, while a correlator channel for the data signal is used for message data. “Measurement data” refers to the measurement data that the receiver generates regarding the signal timing. This data is used in the position and velocity computation process in the receiver. This data typically includes code phase information, carrier phase information, and code and carrier frequency information.
In the conventional GPS implementation, the C/A code and the P/Y code are such different and require so different processing rates that using a common processing channel to achieve the reception of both signals is not cost-effective.